The invention relates generally to hybrid pixel arrays used in x-ray imaging; and in particular, to non-destructive real-time examination of organic and inorganic subjects in biomedical applications and manufacturing processes, respectively.
It is known that conversion devices such as phosphor or scintillating material of some kind to produce visible light from x-rays. The x-rays when received by a solid state device are converted into an electronic signal. Visible light devices are also known which use silicon to convert x-rays directly to an electronic signal. They are fabricated as an individual large detector or as monolithic charge coupled devices (CCDs) having a relatively shallow sensitive region of approximately 10 microns. A single large detector or array of CCDs is suitable only for detecting and imaging x-rays at low energies, below 10 KeV.
There is a need to improve the resolution of existing scintillating and phosphor imagining devices which are relatively insensitive and have low contrast and spatial resolution.
Current x-ray imagining technology is adequate for real-time nondestructive inspection of manufacturing process in only a few specialized cases. Current real-time x-ray systems employ light converter screens or intensifiers that first convert the x-rays to visible light and then view the resulting visible image with conventional or low-light level vidicons or CCD cameras. These known systems suffer from reduced sensitivity and resolution because of the inefficiencies in the light converter screens and the multiple steps between sensing of the x-ray photons and the production of the resultant digital electronic image.
Shortcomings of screen techniques are the loss of efficiency in the process and spatial blurring caused by natural spreading of the fluorescent light as it travels to the detector. Also, spatial resolution and contrast sensitivity are limited not only by the x-ray converter screen, but by the visible observing system.
The charge-coupled device (CCD) is the most common architecture for solid-state image sensors. In a CCD, light absorbed in the silicon produces hole-electron pairs. Most of this charge production at visible wavelengths, is within a few microns of the surface of the detector. The charge diffuses under the influence of a localized electrical field near the surface and is collected on an array of capacitors. The image is read out by sequentially shifting the collected charge along a chain of transistors. Two-dimensional readout is accomplished by arranging a large number of parallel columns to empty into successive ports in a single readout row, whose entire contents are shifted out once per step of the column shift. The signals, being small, are amplified on the chip by a single low-noise amplifier.
One of the primary limitations of CCD's is in the detection of x-rays. In a conventional buried channel CCD, the electric fields in the immediate vicinity of the front gate array are responsible for binding and transferring charge form one storage site to the next. These fields do not extend far into the bulk substrate silicon. They are not able to efficiently capture charge diffusing from distances greater than the inter-gate distance.
This limitation is not particularly significant in optical image sensing where the penetration depth of light is only a few microns, or in the soft X-ray band, where it is a fraction of 1 micron.
It is difficult to fabricate a CCD which combines readout and detection in the same layer of silicon. The detector chip needs a high-resistivity low-doping-concentration substrate, while the readout chip is best implemented with a low resistivity medium.
There is a need for real-time nondestructive inspection of manufacturing processes. A real-time system would allow x-ray imaging of flaws, defects, and hidden features of manufactured products. This capability will greatly improve the monitoring and control of a wide variety of manufacturing processes.
Industrial applications are hampered by the cost of film and long exposure times. Phosphor-based electronic arrays and image intensifiers produce low-resolution images. These drawbacks limit x-ray inspection in manufacturing to the most critical high-end applications such as turbine blade inspection. Fast-action high-resolution x-ray vision applied to high-production casting lines and soldered circuit boards would greatly enhance manufacturing efficiency and quality of the products. Existing inspection methods are not conducive to closed-loop control of such processes as laser welding.